The present invention relates to a porous super-abrasive grinder or whetstone for use in the field of precision machining. More particularly, the present invention relates to a porous super-abrasive grinder that ensures highly efficient work and has superior strength, and a method of manufacturing the grinder.
Grinding (abrasive) particles of diamond and cubic boron nitride (hereinafter also referred to as xe2x80x9ccBNxe2x80x9d) are called xe2x80x9csuper-abrasive particlesxe2x80x9d because of having very high hardness, and are often used in precision grinding of steel, very hard metals, glass, ceramics, and stone materials. A super-abrasive grinder (hereinafter simply referred to as xe2x80x9cgrinderxe2x80x9d) using such super-abrasive particles is generally manufactured by binding the super-abrasive particles together by a binder and molding them into a desired shape. Depending on types of binders used, there are a resin bond grinder using a synthetic resin, a vitrified bond grinder using a vitreous material, and a metal bond grinder using a metal. These grinders are selectively employed in accordance with characteristics of works to be ground. Recently, with an increased density of devices and more widespread use of those devices as represented by integrated circuits employing thin film processes, it has been required from the economical reason to precisely grind a work to such an extent that a width of grinding allowance for a substrate is, e.g., not larger than 0.3 mm. A thin-edge grinding wheel capable of achieving the above grinding has been demanded correspondingly.
Of the above grinders, the metal bond grinder is manufactured by putting metal powder including abrasive particles scattered uniformly therein into a mold together with a metal base, and subjecting it to pressing and sintering (or hot pressing) processes. The binder of metal used in the metal bond grinder uses, for example, a Cuxe2x80x94Sn system, a Cuxe2x80x94Snxe2x80x94Co system, a Cuxe2x80x94Snxe2x80x94Fexe2x80x94Co system, a Cuxe2x80x94Snxe2x80x94Ni system or a Cuxe2x80x94Snxe2x80x94Fexe2x80x94Ni system or any of these systems to which phosphorus is added. Such a conventional metal bond grinder has an extremely strong binding strength as compared with conventional resinoid and vitrified bond grinders, and is therefore advantageous in exerting a sufficient abrasive-particle retention force required to perform strong grinding by means of super-abrasive particles. In the metal bond grinder, however, the strength and stickness of the binder itself are so high that the binder is not worn during the grinding process. Even when abrasive particles are worn, the abrasive particles cannot fall from the binder. This means that the dressing interval must be shortened and highly sufficient grinding is impossible. Accordingly, the conventional metal bond grinder has the following disadvantages. Since discharging of chips is deteriorated and loading occurs easily, the grinding resistance increases and the grinding quality deteriorates, so that the heat generated is increased. Further, the grinder has a tendency to unsuccessfully finish the surface of a work. It is therefore very difficult to perform grinding with high efficiency by increasing the infeed or increasing the contact area of the grinder and the work. In addition, the metal bond is softened to cause plastic deformation upon grinding, and loading takes place in the surface of the grinder.
Heretofore, most of thin-edge grinders for use in the precision grinding have been metal bond grinders from the viewpoint of strength. The metal bond grinder is manufactured by the electro-forming or sintering method using, as a binder, a Ni- or bronze-base alloy. However, the structure of a binder phase is dense and a difficulty is encountered in dressing the metal bond grinder. An intricate and expensive technique and apparatus employing the electrolytic method, etc. have been therefore required. To activate a grinder, it is required to project an edge of super-abrasive particles from the surface of the binder phase. Generally, a grinder just after being formed has a condition where the super-abrasive particles and the binder phase are at the same level in the surface of the grinder. To project an edge of the super-abrasive particles from such a condition, a surface layer of the binder phase must be removed to a certain depth while leaving the super-abrasive particles. This operation is called xe2x80x9cdressingxe2x80x9d. If the surface layer of the binder phase is flat, it is very difficult to remove only the surface layer of the binder phase by a scraping or similar method, for example, while leaving the super-abrasive particles. This means the necessity of an intricate and expensive method, such as the electrolytic method, for ablating the surface layer of the binder phase.
On the other hand, a vitrified bond grinder is usually manufactured by molding a mixture of ceramic particles as a binder and super-abrasive particles, and sintering the molded mixture under pressure. Since a binder phase is porous and has a coarse structure, special dressing is not required. Also, since grinding chips generated during the grinding work are captured in pockets formed by pores and then discharged, loading does not easily occur. Further, even when an edge of the super-abrasive particles is worn, the binder phase is so coarse and brittle as to fall off in an appropriate manner. As a result, a new edge appears and glazing does not also easily occur. In the vitrified bond grinder, however, the binder phase is brittle and the bonding force between the binder and the super-abrasive particles is weak. Accordingly, the vitrified bond grinder cannot be formed into a grinder having a thin edge with a thickness of, for example, not greater than 0.3 mm, and the edge is easily susceptible to dulling. The vitrified bond grinder is therefore not economical when used to grind a difficult-to-grind work having high hardness under a strong pressure, because of serious wear.
In order to eliminate the above defects, a continuous porous metal bond grinder is proposed (Japanese Unexamined Patent Application Publication No. 59(1984)-182064). However, this metal bond grinder does not utilize the powder sintering method. More particularly, the Publication discloses a manufacturing method as follows. An inorganic compound that is melted by a solvent is sintered into a desired shape. Thereafter, voids in the sintered body are filled with abrasive particles and the sintered body having voids filled with abrasive particles is preheated. A melted metal or alloy is pressed into the voids of the sintered body filled with the abrasive particles and is then solidified. Subsequently, the inorganic compound is liquated out by a solvent. Thus, the disclosed method is to add, as filler, a pore forming agent and to form pores in a layer of the abrasive particles. Further, various measures for preventing a reduction in grinding quality have been proposed. In one example, many layers of metal coatings are formed on abrasive particles, and the coated abrasive particles are sintered by hot pressing so as to have a structure that is like a vitrified bond and includes pores formed therein (Japanese Examined Patent Application Publication No. 54(1979)-31727). Furthermore, a grinder using cast iron for the purpose of preventing loading of the grinder has been proposed (Japanese Unexamined Patent Application Publication No. 3(1991)-264263). The grinder using cast iron as a bond advantageously has great strength and high rigidity, enables heavy grinding to be performed at a high infeed, and is worn in the brittle fracture manner without the occurrence of plastic deformation, so that loading is less likely to occur. However, the bond of this grinder is too strong and accordingly the dressing property is deteriorated as compared with the bond of the copper system. Additionally, because of the high rigidity, it is difficult at the present to practically employ this grinder with the existing grinding machines and methods. By forming a large number of pores within the layer of the abrasive particles, a grinding liquid can be impregnated into the pores to enhance the cooling characteristics of the grinder, and the grinding resistance can be made small by the pores to improve the grinding quality. In other words, it can be expected that less heat is generated and the surface of a work is finished with high quality. However, when a large number of pores are formed in the conventional copper-system metal bond grinder, the strength and the abrasive- particle retention force are naturally reduced, so that the sufficient grinding performance cannot be obtained.
Moreover, in a grinder using non-porous cast iron as a bond, iron powder is added to cast iron powder because of the inferiority of the sintering characteristics of the cast iron powder, and a powder mixture is molded with the load of 8,000 kgf/cm2 to 10.000 kgf/cm2. With addition of the iron powder, the original brittle fracture characteristic of the cast iron is lost and plastic deformation is apt to occur in the same manner as the copper system bond. As a result, the characteristics of the cast iron are not utilized sufficiently. Additionally, if the abrasive particles directly contact the cast iron, diamond is lost upon reaction of iron and carbon. It is therefore required to coat diamond with a film for protection.
Taking into account the above-described state of the art, the inventors have accomplished an invention wherein pores are formed in the structure of a metal bond grinder to provide a porous structure, with the view of realizing a grinder that has great strength and a high binding force between a binder and super-abrasive particles (Japanese Unexamined Patent Application Publication Nos. 7(1995)-251378 and 7(1995)-251379). This porous metal bond grinder can be manufactured, for example, by mixing super-abrasive particles and binder metal particles together, compressing a mixture into a shape of the grinder with or without a heat-developing binder, and sintering a compressed body under such a temperature and pressure that the binder metal particles are bonded to each other while maintaining the particulate form, and the binder particles and the super-abrasive particles are bonded to each other. The porous metal bond grinder thus manufactured has been practiced with fairly satisfactory results because of the following advantages. The bonding force between the binder and the super-abrasive particles is strong, and the dressing property is good. Grinding chips, etc. generated during the grinding work are captured in pockets formed by pores and then discharged; hence loading does not easily occur. Further, even when an edge of the super-abrasive particles is worn, the binder phase is caused to fall off in an appropriate manner as a result of properly adjusting the sintering strength of the binder phase, so that a new edge appears and glazing does not also easily occur.
In the above porous metal bond grinder, however, the bonding force between the super-abrasive particles and the binder is strong, but the strength is within the range obtainable with a metal. Further, since the binder phase also includes a porous metal, there is a limitation in value of the Young""s modulus. Thus, although the above metal bond grinder has succeeded in remarkably improving the grinding performance as compared with the existing grinders, problems still remain in that there is a room of improvement in the reaction between the super-abrasive particles and the binder and the material physical properties of the binder phase itself.
To overcome the above-mentioned problems, the inventors have conducted studies with intent of enhancing the bonding force between super-abrasive particles and a binder, increasing attrition of the binder during a grinding process, and improving physical properties of a grinder.
An object of the present invention is to provide a porous abrasive-particle grinder and a method of manufacturing the grinder, in which the bonding force between super-abrasive particles and a binder is strong, dressing, dulling, loading and glazing properties are improved in a well-balanced way, and the grinder has strength enough to be used as a thin-edge grinder for fine grinding.
The present invention has been made for achieving the above object, and will be described below in more detail.
The present invention resides in a porous abrasive-particle grinder comprising super-abrasive particles as grinding particles and metal powder as a binder, wherein the binder is formed into a porous body holding the super-abrasive particles with chemical and physical bonding, and at least the surface of the formed porous body is denatured to ceramic. Since the binder is formed into a porous structure phase having adjusted porosity and at least the surface of the formed porous body is denatured to ceramic, the porous abrasive-particle grinder has such characteristics that the bonding force between the super-abrasive particles and the binder is strong, dressing, dulling, loading and glazing properties are improved in a well-balanced way, and the grinder has strength enough to be used as a thin-edge grinder for fine grinding.
The abrasive particles are selected from a group consisting of materials with the Knoop hardness of not lower than 1000. More specifically, the abrasive particles are selected from a group consisting of diamond and cubic boron nitride. The super-abrasive particles have a mean particle size of not greater than 1000 xcexcm.
The binder comprises a metal capable of chemically and physically bonding to the super-abrasive particles under heating, and the porous body has a porous structure phase formed by powder sintering. The above metal is one or more selected from a group consisting of Fe, Cu, Ni, Co, Cr, Ta, V, Nb, Al, W, Ti, Si and Zr. Porosity of the whole of the grinder is 5 to 60%, preferably 5 to 45%.
The present invention resides in a method of manufacturing a porous abrasive-particle grinder by using, as raw materials, super-abrasive particles as grinding particles and metal powder as a binder, wherein protrusion of the abrasive particles and grip of the abrasive particles are controlled separately.
Also, the present invention resides in a method of manufacturing a porous abrasive-particle grinder by using, as raw materials, super-abrasive particles as grinding particles and metal powder as a binder, wherein protrusion of the abrasive particles is first controlled and then grip of the abrasive particles is controlled.
The present invention resides in a method of manufacturing a porous abrasive-particle grinder, the method comprising the steps of mixing super-abrasive particles as grinding particles and metal powder as a binder together, molding a mixture into a predetermined size and shape, sintering a molding under temperature and pressure adjusted such that atoms are diffused at the interface between the super-abrasive particles and binder particles in the molding and the binder particles are sintered together into a porous body, and heating a sintered body in the presence of one or more kinds of gases selected from a group consisting of nitrogen, carbon and hydrogen so that at least the surface of the porous body is denatured to ceramic.
Super-abrasive particles having a mean particle size of not greater than 1000 xcexcm are employed as the grinding particles. Super-abrasive particles selected from a group consisting of materials with the Knoop hardness of not lower than 1000 are employed as the grinding particles. Diamond and cubic boron nitride are employed as the materials with the Knoop hardness of not lower than 1000.
A metal capable of chemically and physically bonding to the abrasive particles under heating is used as the binder, and a porous body having a porous structure phase is formed by powder sintering. The above metal is one or more selected from a group consisting of Fe, Cu, Ni, Co, Cr, Ta, V, Nb, Al, W, Ti, Si and Zr. The sintering step is performed under temperature and pressure adjusted such that porosity of the whole of the grinder is 5 to 60%. Preferably, the sintering step is performed under temperature and pressure adjusted such that porosity of the whole of the grinder is 5 to 45%. The sintering step is performed by an electro-sintering process, and temperature and pressure in the sintering step are respectively in the range of 600xc2x0 C. to 2000xc2x0 C. and in the range of 5 MPa to 50 MPa. Alternatively, the sintering step is performed by a hot-press sintering process, and temperature and pressure in the sintering step are respectively in the range of 600xc2x0 C. to 2000xc2x0 C. and in the range of 5 MPa to 50 MPa. Any other suitable sintering methods such as atmosphere sintering and HIP sintering are also usable.